A post-deflection accelerating and scan expansion lens system is included in a cathode ray tube to perform two distinct functions. The lens system magnifies the amount of electron beam deflection produced by the deflection means to provide an image of the desired size on the fluorescent screen. The lens system also increases the velocity of the electrons in the electron beam by means of a high intensity electric field to raise the energy of the electrons and thereby produce a brighter image on the fluorescent screen.
A number of accelerating and scan expansion lens systems make use of a focusing lens of the quadrupole type. For an electron beam traveling toward a fluorescent screen in the Z direction and deflected horizontally in the X direction and vertically in the Y direction of a three dimensional Cartesian coordinate system, a quadrupole lens converges with respect to its principal axis the beam in one of the X-Z and Y-Z planes and diverges it in the other one of the planes. The particular planes of convergence and divergence are determined by the distribution of the voltages applied to the quadrupole lens electrodes. Thus, the paths of deflected electron beams traveling in the Z direction and converging in the Y-Z plane are brought to a line focus parallel to the X axis. To obtain a point focus of the electron beam on the fluorescent screen and thereby produce an image in sharp focus and great detail, a post-deflection lens system incorporating a quadrupole lens operating in the manner described requires the use of a second quadrupole lens which converges the electron beam in the Y-Z plane and diverges it in the X-Z plane.
Two distinct distortion mechanisms associated with quadrupole accelerating and scan expansion lens systems deform the image displayed on the fluorescent screen. These include nonlinear magnification of the beam deflection angle and the "pincushion" type of geometry distortion. Nonlinear magnification of the beam deflection angle is produced by the nonuniform influence of the electric field flux lines of the lens system on the direction of beam travel. The electric field expands the scan deflection angle of the beam to produce a corresponding light image of the desired size on the fluorescent screen, Generally, in lens systems of this type, an electron beam which is deflected to a great degree is not magnified in the same proportion relative to a beam deflected to a substantially lesser degree. For example, a low voltage sine wave represented in the time domain would appear on a fluorescent screen as being frequency modulated at the end points of the display because the nonlinear effects of scan expansion on the time base sweep would produce zero crossings at the ends of the sign wave image which would be different from the uniform spacing of the zero crossings at the center portion of the display.
Geometry distortion of the displayed image is caused by aberrations in the shape of the electric field flux lines in the space above and below the lens axis. The electric field flux line pattern developed in the X-Y plane in a quadrupole lens system is characterized generally as having a plurality of parallel horizontal lines disposed transversely of an electron beam traveling along the Z axis. Moderate fluctuations in the flux lines produce geometry distortion of the image. Such distortion is characterized by deformations in an image of intended rectangular form which is displayed as having stretched vertices and concave side portions. An intolerable degree of geometry distortion of an image generally is introduced in short length cathode ray tubes wherein the quadrupole accelerating lens with a short focal length provides a high intensity electric field for electron beam deflection angle magnification.
A short length cathode ray tube is a tube that performs comparably to a tube of standard length but has an overall length of about five centimeters less than the standard length.
The basic principles underlying the operation of electrostatic lenses for focusing electron beams produced in electron discharge devices are described in U.S. Pat. No. 2,412,687 of Klemperer. The Klemperer patent teaches the formation of an electron lens that makes use of a pair of aligned tubular electrodes maintained at different potentials to converge an electron beam toward the electrode which is held at a more positive potential. One lens system described by the Klemperer patent includes two overlapping coaxial cylindrical electrodes with portions projecting from the end of the smaller diameter electrode being embraced by the larger diameter electrode. The Klemperer patent does not suggest the use of such lens systems for acceleration of a deflected electron beam and, therefore, does not disclose compensating means for correcting nonlinear magnification of the amount of deflection of the beam and geometry distortion of the corresponding light image displayed on a fluorescent screen.
A quadrupolar accelerating electrostatic lens system which includes a "lipped" cylindrical tube protruding into a wider tube, such as the conductive wall coating on the neck of a cathode ray tube envelope, is described in O. Klemperer, Electron Optics, 100-106 (3d ed. Cambridge University Press, 1971). In Electron Optics, Klemperer discusses in general the parameters associated with focusing an electron beam for accomplishing scan magnification to produce increased deflection of an electron beam, but does not address the problems of nonlinear scan expansion of the electron beam or geometry distortion of the image displayed on a fluorescent screen.
U.S. Pat. No. 3,496,406 of Deschamps describes a cathode ray tube having an electrostatic lens system that includes a quadrupole scan expansion lens disposed within a dome-shaped post-deflection acceleration electrode having a slot at its apex. The dome-shaped electrode is positioned to enclose the portion of the quadrupole lens facing the funnel portion of the cathode ray tube envelope that bears a conductive coating to which is applied the accelerating potential. The dome-shaped electrode is held at ground potential, thereby providing a shield to isolate the quadrupole lens from the effects of the intense electric field developed between the dome-shaped electrode and the conductive coating on the inner surface of the tube. This combination of the scan expansion quadrupole lens and the dome-shaped electrode constitutes a lens system which causes the electron beam paths to cross over in the vertical plane and the electrons to be accelerated toward the fluorescent screen after they exit the slot in the dome-shaped electrode.
A discussion of the operation of and the mathematical expressions relating to a short length cathode ray oscilloscope tube having a quadrupole scan expansion lens in conjunction with a dome-shaped electrode accelerating system of the type disclosed in the Deschamps patent is described in A. Martin & J. Deschamps, A Short Length Rectangular Oscilloscope Tube With High Deflection Sensitivity By Using an Original Iechnique, 12 Proceedings of the Society for Information Display 18 (1st Qtr. 1971). However, there is no disclosure of a compensating means for correcting distortion of the image displayed on the screen.
U.S. Pat. No. 3,792,303 of Albertin, et al. describes a modification of the Deschamps lens system in an attempt to correct for distortion of the displayed image. Albertin, et al. increases the length of the sides of the dome-shaped electrode to cover all but one side of the quadrupole scan expansion lens. A single disk-shaped slot electrode is disposed perpendicular to the electron beam axis on each end of the quadrupole lens. The first slot electrode is positioned inside the dome-shaped electrode beyond the quadrupole lens and in front of the slot in the dome-shaped electrode. This slot electrode is physically and electrically connected to the dome-shaped electrode so that both electrodes are at ground potential. The second slot electrode is positioned adjacent the edge of the base portion of the dome-shaped electrode in front of the quadrupole lens and is electrically isolated so that it can be raised to a voltage which is different from that of the first slot and dome-shaped electrodes.
A disk-shaped electrostatic screen or shield electrode having a conventional rectangular slot is held at ground potential and is placed immediately in front of the second slot electrode of Albertin, et al. The screen electrode and dome-shaped electrode substantially fully enclose the quadrupole lens within an equipotential space at ground potential.
Albertin, et al. describes a compensation technique which is said to correct for image distortion by separating into horizontal and vertical components the combined effects of scan nonlinearity and geometry distortion. The geometry of the aperture of the first slot electrode included within the dome-shaped electrode is determined experimentally to correct for distortions which appear on the screen and are introduced by the horizontal deflection of the electron beam during scanning. The geoxetry of the aperture of the second slot electrode positioned in front of the quadrupole lens is determined experimentally in conjunction with a suitable applied potential to correct for distortions which appear on the screen and are introduced by the vertical deflection of the electron beam during scanning. In addition, the voltage applied to the second slot electrode affects the performance of the first slot electrode. Thus, the geometry of the aperture of the first slot electrode corrects for distortion introduced by not only the horizontal electron beam trace, but also the presence of the second slot electrode.
This compensation technique suffers from a disadvantage in that experimental adjustments for the horizontal and vertical image distortion components are not independent. Thus, the aperture size and the voltage applied to the second slot electrode affect the aperture size and compensating electric field produced by the first slot electrode.
U.S. Pat. Nos. 4,137,479 and 4,188,563 of Janko describe a cathode ray tube having a post-deflection quadrupole lens system which, unlike the quadrupole lenses disclosed in Deschamps and Albertin, et al., simultaneously expands the deflection scan of the electron beam and accelerates the electrons toward the fluorescent screen. The Janko lens system includes a pair of aligned tubular entrance and exit electrodes of the same diameter, the adjacent ends of which are spaced apart with an air gap therebetween and have interdigitated sections that describe complementary curvilinear courses along the peripheries of the electrodes. The accelerating electric field is produced by connecting the entrance electrode to ground potential and applying the acceleration voltage to the exit electrode which is electrically connected to the conductive coating on the funnel portion of the tube. An octupole lens system is positioned in front of and adjacent to the entrance electrode to correct for both nonlinear scan magnification of the electron beam and geometry distortion of the displayed image. Janko also suggests an alternative embodiment which utilizes a pair of coaxial tubular electrodes of different diameters with the outer electrode encompassing the curvilinear edge portions of the inner electrode.
The Janko lens system of aligned tubular electrodes of the same diameter preceded by an octupole distortion correction lens is of limited utility as an accelerating lens because of a tendency of dielectric breakdown to occur in the air gap between the electrodes. Since a potential difference of about 23 kv is applied between the electrodes, the adjacent edges of each electrode must be smoothed to eliminate sharp edge points of microscopic dimensions which tend to produce excessively high electric field strengths that cause field emission of electrons which form an electric arc between the electrodes.
The coaxial lens system of the alternative embodiment suggested by Janko is less susceptible to dielectric breakdown, but it has the inherent characteristic of producing a weaker lens which provides less scan magnification of the electron beam deflection angle. In general, for a given applied voltage, the focal length of such a lens is directly proportional to the diameter of the inner electrode. Thus, an inner electrode of relatively small diameter is required to produce a strong lens with a short focal length. A strong scan expansion lens enhances geometry distortion effects, and it has been determined empirically that the Janko coaxial accelerating lens operating with an octupole correction lens produces geometry distortion to an unacceptable degree for inner electrode diameters of less than 1.905 centimeters. Thus, the Janko coaxial lens system is not suitable for use in short length cathode ray tubes, in which strong scan expansion lenses with short focal lengths are required.
U.S. Pat. No. 4,142,128 of Odenthal describes a cathode ray tube provided with a rectangular box-shaped scan expansion lens which includes at least four tubular elements disposed end-to-end and spaced apart to isolate them electrically. The tubular elements have bias voltages of different values that change the lens characteristics. Distortion of the image due to nonlinear scan expansion is corrected by the inclusion of additional side plates to which a differential bias voltage is applied.
U.S Pat. No. 3,023,336 of Frenkel describes a cathode ray tube in which post-deflection acceleration and scan expansion is accomplished by a combination of an electrostatic accelerating and converging lens with a magnetic converging lens which create spherical aberrational effects that compensate for each other to project an image in sharp detail on a fluorescent screen.